1. Field of the Invention
The present invention relates to an image forming apparatus in which a toner image formed electrostatically on an image bearing member is transferred onto a transfer material.
2. Related Background Art
In the conventional color image forming apparatus based on electrophotographic process, there is known a configuration provided with an intermediate transfer member in addition to a photosensitive member. In such configuration, there is repeated plural times so-called primary image transfer in which a toner image formed on the photosensitive drum is transferred onto the intermediate transfer member to superpose toner images of plural colors thereon, and then there is executed secondary image transfer in which such toner images of plural colors are collectively transferred onto a transfer material such as paper.
FIG. 6 shows an example of the image forming apparatus employing an intermediate transfer belt as the image bearing member.
Around a photosensitive drum 101 supported rotatably in a direction R1, there are provided four developing units 105, 106, 107, 108 respectively containing toners of four colors, namely black (BK), cyan (C), magenta (M) and yellow (Y). These developing units are so constructed that one unit therein, used for developing the electrostatic latent image present on the photosensitive drum 101, is brought into abut thereon by contact means (not shown).
The photosensitive drum 101 is uniformly charged by a charger 102, and an electrostatic latent image is formed by a scanning light (laser beam) 104 emitted by a laser exposure light source 103. The electrostatic latent image is then developed, by toner deposition by the above-mentioned developing units 105 etc., into a toner image, which is transferred, by a primary image transfer, onto an intermediate transfer belt. 109 at a primary transfer nip N.sub.1 with a primary transfer roller 110. The formation, development and primary transfer of the electrostatic latent image are repeated with the toners of four colors, employing the developing units 108, 107, 106, 105, in the order of Y, M, C and BK, whereby four superposed color toner images are formed on the intermediate transfer belt 109. Then, these toner images are collectively transferred, in a secondary image transfer, onto a transfer material 118 which is pinched and conveyed at a secondary transfer position N.sub.2 formed between a secondary transfer roller 111 and the intermediate transfer belt 109.
In the following the primary and secondary transfers mentioned above will be explained in more details. In case the photosensitive drum 101 is composed of a negatively chargeable OPC (organic photoconductor) photosensitive member, negative toners are employed for developing the areas, exposed to the laser beam 104, in the developing units 105 to 108. Thus a positive transfer bias voltage is applied by a bias electrode 120 to the primary transfer roller 110.
The intermediate transfer belt 109 can be composed, for example, of an endless resinous belt of a thickness of about 100 to 300 .mu.m, of which resistance is adjusted to a volumic resistivity of 10.sup.11 to 10.sup.16 .OMEGA..cm. The resinous belt can be composed, for example, of a resinous film (subjected to resistance adjustment if necessary) of PVDF (polyvinylidene fluoride), nylon, PET (polyethylene terephthalate) or polycarbonate. As another example, the above-mentioned resinous belt may adjusted to a volumic resistivity of 10.sup.7 to 10.sup.11 .OMEGA..cm with conductive filler material such as carbon, ZnO, SnO.sub.2 or TiO.sub.2. A medium to low resistance level in the latter example allows to prevent image defects resulting from charge accumulation on the intermediate transfer belt 109.
As still another example, the intermediate transfer belt 109 can also be composed of a rubber material (chloroprene rubber, EPDM, NBR or urethane rubber) of a lower hardness, having a thickness of about 0.5 to 2 mm and adjusted to a volumic resistivity of 10.sup.6 to 10.sup.11 .OMEGA..cm.
Such intermediate transfer belt 109 is supported by a backup roller 112, a drive roller 115 and a tension roller 116. The primary transfer roller 110 is generally of a low resistance, having a volumic resistivity of 10.sup.5 .OMEGA..cm or less. In the above-described configuration, the primary transfer roller 110 and the bias voltage source 120 constitute primary transfer means.
Then the toner images are subjected to the secondary transfer onto the transfer material 118, by secondary transfer means consisting of a secondary transfer roller 111, the backup roller 112 and the bias voltage source 120. The secondary transfer is executed by positioning a backup roller 112 of a low resistance, grounded or suitably biased, as a counter electrode inside the intermediate transfer belt 109, forming a secondary transfer nip N.sub.2 between the backup roller 112 and an outside secondary transfer roller 111 of a low resistance, across the intermediate transfer belt 109, applying a positive transfer bias to the secondary transfer roller 111 by the bias voltage source 120 and abutting the secondary transfer roller 111 from the rear face side of the transfer material 118.
After the primary transfer mentioned above, the photosensitive drum 101 is subjected to the removal of the toner, remaining after the primary transfer, by a cleaner 119, then to the removal of retentive charge by an exposure device 117, and is used again for the next image formation.
On the other hand, the intermediate transfer belt 109 after the above-mentioned secondary transfer is subjected to the removal of the toner, not transferred to the transfer material 118 but remaining on the intermediate transfer belt 109, by a cleaner 113 and, if necessary, to the charge elimination by a charge eliminator (charge eliminating means) 114.The charge eliminator 114 usually utilizes AC corona charging. For improving the efficiency of charge elimination, an electrode is preferably provided inside the intermediate transfer belt 109.
The above-described charge eliminator 114 may be dispensed with in case the intermediate transfer belt 109 is of medium to low resistance as explained in the foregoing.
The intermediate transfer member can also be formed as a drum-shaped intermediate transfer dram instead of the intermediate transfer belt 109 explained in the foregoing, but, in comparison with such intermediate transfer drum, the intermediate transfer belt 109 is generally superior in the larger freedom of arrangement and in the better separation of the transfer material 118 after the secondary transfer (possibility of separation by curvature).
On the other hand, the intermediate transfer drum can simplify the structure, in comparison with the belt drive required for the intermediate transfer belt. The configuration of such intermediate transfer drum will not be explained further, as the resin or rubber layer formed on the surface of such drum can have electrical characteristics similar to those in case of the intermediate transfer belt.
In the above-described apparatus, the images with the toners of four colors Y, M, C and BK are conventionally formed at first with three colors Y, M, C in an arbitary order and with the black color BK at last.
Such order of image formation has been widely adopted since the multiple transfer process, which precedes the intermediate transfer process and in which the images are transferred in succession onto a transfer material such as paper wound on a rotatable transfer drum. As the black image generally has a larger amount of information such as characters in comparison with the images of other colors, the black toner, if primary transferred as the first to third colors, may re-transferred to the photosensitive member at the primary transfer of the subsequent color thereby inducing a loss of the black toner as a result of such re-transfer, and the above-mentioned order has been adopted to avoid such phenomenon. For this reason, after the secondary transfer of the toners onto the transfer material 118, the BK toner is present at the lower most layer (closest to the surface of the transfer material).
The conventional configuration described above has been associated with the following drawbacks. At the successive primary transfer of the color toner images from the photosensitive drum 101 to the intermediate transfer belt 109, the toner of the first color transferred onto the intermediate transfer belt 109 is retained thereon during the successive transfer of the toners of second to fourth colors, and, during such transfer of the toners of second to fourth colors, there takes place a charge exchange between the toner of the first color and the photosensitive drum 1 or the intermediate transfer belt 109, whereby, at the end of the primary transfer of the toner of the fourth color, the charge or tribo (triboelectricity) of the toner of the first color becomes different from that at the primary transfer of the toner of the first color. As a result, the secondary transfer condition becomes different for the toners of the first to fourth colors present on the intermediate transfer belt, particularly between the toners of the first and fourth colors. As a result, in the above-described configuration, the defective secondary transfer may result in the BK toner of the fourth color if the bias of the secondary transfer roller 111 is matched with the Y toner of the first color for obtaining a higher transfer efficiency, or in the Y toner of the first color if the bias is matched with the BK toner of the fourth color, or there may result a change in the color hue due to a lowered transfer efficiency.
In order to avoid such phenomena, there is conceived a method of so-called post-charging in which the triboelectric charges of the toners of the first to fourth colors on the intermediate transfer belt are re-charged to a substantially same level, prior to the secondary transfer, by a corona charger 122 receiving a DC voltage and an AC voltage, but such methods not only involves a more complex structure of the apparatus and a higher cost but also encounters difficulty in bringing the tribo of the toners of the first to fourth colors to a uniform charge level, thereby resulting in a defect such as image unevenness in the highlight portion in a halftone image.
Also there has been considered a method of recovering the toner, remaining on the intermediate transfer belt 109 after the secondary transfer, by re-charging such remaining toner to a suitable level and collecting (inverse transferring) such remaining toner to the photosensitive drum 101 through the primary transfer nip N.sub.1, but, if such recovery is not executed in a particular rotation step of the drum but at the primary transfer of the toner of the first color in the next image formation, such recovery cannot be achieved satisfactorily if there is employed a high bias voltage for such transfer of the first color. If non-magnetic toner, showing a high charge amount per unit area, is employed as the toner of the first color, the efficiency of primary transfer is lowered if the primary transfer bias is lowered according to the condition of the above-mentioned cleaning.